This invention relates to the equalisation of optical signals in an optical communications system and in particular but not exclusively to the equalisation of wavelength division multiplexed optical signals.
The control of optical power levels in optical communications systems is critical in obtaining optimum performance since the power level of each and every. channel should be sufficient to establish a signal to noise ratio which will provide an acceptable bit error rate, without the power level exceeding a level at which limiting factors such as the onset of non-linear effects result in degradation of the signal. In wavelength division multiplexed (WDM) transmission, it is desirable to maintain the same BER/SNR for each channel, which usually means maintaining each of the power levels of the individual wavelength components at substantially the same level. U.S. Pat. No. 5,513,029 discloses a method of monitoring component power levels in WDM transmission using orthogonal low frequency dither signals and controlling component signal power to maintain optimum performance.
It is also known from GB2314714A that an imbalance of component signal powers in a WDM transmission is likely to occur at an optical amplifier stage, as used to boost signal power at stages in a long distance transmission, utilising optical amplifiers such as erbium doped fibre amplifiers. Such amplifiers have a non-uniform gain characteristic as a function of wavelength which is variable in dependence on the amplifier gain, this change in gain characteristic consequent on change of gain being commonly referred to as dynamic gain tilt.
There is therefore a need to provide optical filtering which is adaptive and which can be used in conjunction with optical amplifiers, or otherwise, in order to maintain a preferred spectral profile of an optical signal.
It is known from Huang et al, IEEE Photonics Technology Letters, Sep. 1996 pp 1243-1245, to provide an acousto-optic tunable filter for dynamic equalisation of channel powers. A disadvantage of such a method is that the filters suffer from polarisation sensitivity and severe channel cross talk.
It is also known from Gobel et al, IEEE Photonics Technology Letters, Mar. 1996, pp 446 to 448, to provide a WDM power level compensator in which demultiplexed channels are subject to power control in respective erbium doped waveguides. A disadvantage of this arrangement is that significant distortion of the modulated optical signal occurs.
It is also known from Madsen et al, IEEE Journal of Lightwave Technology, Mar. 1996, pp 437 to 447, to provide fixed (non-adaptive) filters using a sequence of concatenated Mach-Zehnder interferometers in a planar waveguide structure. Such structures require lengths which are difficult to fit onto a single planar waveguide structure and which have an inherent high insertion loss.
Parallel structures on planar waveguides are known from Dragone, IEEE Photonics Technology Letters, September 1991, pp 812 to 815, which provide non-adaptive filtering with an output at a single wavelength.
It is also disclosed by S. Day in co-pending application U.S. Pat. No. 08/997,752 to provide a variable optical attenuator by means of localised heating of a waveguide.
Yamada et al, Electron Letters 1995, 31, pp 360 to 361, discloses a multiplexer using planar waveguide technology and in which a waveguide array is provided with heating strips for each waveguide in order to compensate for phase errors occurring during fabrication. After such compensation, light components passing through the arrayed waveguides are delayed by respective amounts which differ by a constant phase difference between adjacent waveguides to that recombination in a star coupler at the output of the waveguides is dispersive in wavelength to provide separation of the WDM channels, this arrangement thereby being termed an arrayed waveguide grating.
There is also a further problem that in all of these prior art situations a knowledge of the gain profile of the amplifier to be equalised is required. Thereafter, one or other method of attempting to equalise the gain in the amplifier for different input wavelengths is applied to the output of the amplifier. However, the gain profile is not static and as it changes, for example as the amplifier warms or if a channel is dropped in a WDM system, then the effectiveness of the prior art equalisation techniques is altered. Also, these devices also require the use of a channel monitoring device which generally costs more than the equalisation device.
Therefore, there remains a need to provide an improved yet structurally simple optical equaliser, particularly for use in the context of correcting gain tilt in optical amplification stages of a communications system.
It is an object of the present invention to provide optical equalisation to, at least partially, compensate for the effects of gain tilt in optical amplifiers.
It is a further object of the present invention to provide an adaptive optical equaliser using planar waveguide technology, the operation of which is technically simple in comparison with prior art optical equalisers.
According to a first aspect of the present invention there is disclosed an optical equaliser, for use in an optical communications system, comprising a plurality of concatenated, fixed modulation depth, periodic filters each of which may be individually dynamically wavelength tuned in order to optimise optical equalisation of an optical signal transmitted by an optical element in the optical communications system.
Preferably, each periodic filter has a fixed period with frequency.
Most preferably, each periodic filter comprises a gain control means, comprising a means for measuring the power output of said element of the optical communications system for a given filter wavelength, and means of tuning the filter wavelength in order to minimise output power.
Preferably, the means of tuning the wavelength of each filter is an analogue dither generator.
Preferably, individual filters are grouped into sets of filters of substantially equal modulation depth.
Preferably, each filter within a set has a different fixed period with frequency.
Preferably, the equaliser comprises three sets of three filters arranged in series, the first set of filters having a modulation depth of 4 dB, the next set of filters having a modulation depth of 2 dB and the next set of filters having a modulation depth of 1 dB.
Preferably, each periodic filter is a two arm Mach-Zehnder interferometer.
According to a second aspect of the present invention there is provided a method of optical equalisation of an optical signal, for use in an optical communications system, comprising the steps of:
filtering the optical signal transmitted by said optical element in the optical communications system through a plurality of concatenated, fixed modulation depth, periodic filters; and
dynamically wavelength tuning each of said filter in order to optimise optical equalisation of said optical signal.
According to a third aspect of the present invention there is provided an optical amplifier including an amplifying element and an optical equaliser comprising a plurality of concatenated, fixed modulation depth, periodic filters each of which may be dynamically wavelength tuned in order to optimise the optical equalisation of an optical signal transmitted by said amplifying element.
According to a fourth aspect of the present invention there is provided an optical communications system including an optical equaliser comprising a plurality of concatenated, fixed modulation depth, periodic filters each of which may be individually dynamically wavelength tuned in order to optimise optical equalisation of an optical signal transmitted by an optical element in the optical communications system.
It is an advantage of the present invention that optical equalisation is controlled through adjustment of a single variable (namely, wavelength) for each filter and can be achieved with simple analogue dither techniques local to each periodic filter. Therefore, there is no requirement to monitor channel positions or perform Fourier transforms to provide a complex drive signal for the filters.
Thus, each filter in an equaliser in accordance with the present invention will be much simpler than those in prior art equalisers because the optimisation process is local to each filter and performed on only a single parameter, i.e. wavelength.
Preferred embodiments of the present invention will now be described by way of example only.